The present invention relates to rotary piston machines of trochoidal design, particularly of the epitrochoidal type. In these machines, some form of guide or follow-up drive mechanism is required to ensure that the piston rotates in a controlled manner about a drive shaft in the opposite direction to the rotation of the drive shaft, and the present invention is particularly, but not essentially, concerned with such a follow-up drive mechanism.
The best known version of a rotary engine of trochoidal design of the epitrochoidal type is the Wankel engine, and the widely accepted best solution to the piston follow-up mechanism for the Wankel engine is a 3:2 gear drive. The Wankel engine uses a piston of the epitrochoidal type defined with an inner envelope.
In rotary piston machines of trochoidal design in which the piston substantially conforms to an epitrochoid having 1:1 generating circles, the follow-up drive mechanism must produce a 2:1 ratio. There have been many proposals to produce this type of drive using a gear driven direct kinematic follow-up mechanism for such a machine in which the piston is defined with an outer envelope, but for a given eccentricity or stroke of the piston on the drive shaft the diameters of the gears are fixed and the static gear must have a pitch circle diameter equal to 4 times the eccentricity of the drive shaft. Furthermore, in order to fit the gears and piston to the eccentric part of the drive shaft it is necessary to manufacture the drive shaft in two parts. In practice these requirements mean that a considerable necking down and therefore weakening of the drive shaft may be required in the area of the smaller pinion gear, to such an extent that the reduced strength of the drive shaft may virtually eliminate the possibility of constructing a multiple piston engine. Also, the rapidly fluctuating loads on the gears will tend to reduce the life of the gears, and the accuracy of the positioning of the piston will become unacceptable with increasing gear wear as the piston will then tend to be able to contact the housing.
One proposal avoiding the use of gears in a 2:1 follow-up drive mechanism for an epitrochoidal rotary piston is made in U.S. Pat. Nos. 3,909,163 and 3,923,430 in which an eccentric elongate rotary body is disposed on a continuous drive shaft for rotation therewith, a sleeve is disposed over the rotary body for rotation relative thereto, one or more rotary pistons are mounted on the sleeve for rotation therewith and a single follow-up mechanism is provided on the sleeve for all of the pistons. The follow-up drive mechanism comprises a pair of offset eccenters mounted on the sleeve for rotation therewith and with the or each piston, from which they are axially spaced. The eccenters are rotatable within respective guide mechanisms which are constrained to reciprocate along rectilinear paths by relatively inclined straight rods. This mechanism operates on the principle that any fixed chosen point on the or each piston will trace an elliptical path relative to the housing when the piston rotates with its correct relative motion with respect to the drive shaft and housing, that is at twice the angular velocity of the drive shaft but in the opposite direction. If the chosen point on the piston is spaced from the eccentric axis of the piston by a distance equal to the piston eccentricity from the axis of rotation of the drive shaft, the ellipse traced on the housing by the fixed point on the piston becomes a straight line. This is a special case of the ellipse when its minor axis becomes zero. Therefore, to obtain the required synchronised motion any point on the piston or fixed relative to the piston at a radius equal to the drive shaft eccentricity from the axis on which the piston rotates must be constrained to move in a straight line through the axis of the drive shaft.
While the proposal in the aforementioned U.S. patent specifications alleviates the necking down of the drive shaft which is required with a direct kinematic gear driven follow-up mechanism, it requires very substantial counter balancing to balance the substantial weight of the rotating body eccentrically mounted for rotation with the drive shaft. Furthermore, since the eccentric rotating body and sleeve mounted for rotation relative thereto are continuous through each piston the unbalanced mass of the eccentric rotating body will increase for each additional piston and the distance between the main bearings at each end of the engine may be very large. It is not practical to have bearings between the end bearings. Even for a single piston engine, the spacing between the eccenters and the piston is substantial requiring well spaced drive shaft bearings. Also, for a multiple piston engine of this design, since all of the pistons are mounted on the one continuous sleeve, each trochoidal housing for an associated piston, and the corresponding plugs and ports, must be angularly offset relative to the others, leading to an engine which may be costly and very difficult to build and service.
The proposal in U.S. Pat. No. 4,086,038 uses a similar follow-up drive mechanism to that in the aforementioned U.S. Patent Specifications but the rotating body and sleeve is replaced by a reduced diameter shaft which is journalled for eccentric rotation relative to the divided drive shaft, with the piston or all of the pistons and the associated guide mechanism fixed for rotation with the reduced diameter shaft. All of the pistons must be on the one reduced diameter shaft and, as with the aforedescribed proposal, there is only one follow-up mechanism for all of the pistons which is equally spaced from the pair of pistons illustrated, all of which means an excessive distance between the main bearings of the drive shaft. Furthermore, it is possible for the opposed portions of the drive shaft to move out of phase due to deflection under load which will tend to cause excessive friction and ultimately seizure with the pistons fouling the housing.